Australia's caretaker prime minister, Julia Gillard, has signed a deal with the Greens that will ensure their support for Labor in the country's hung parliament.

One and a half weeks after the election, both Gillard's Labor and the conservative opposition are still three seats short of an overall majority, with four independent MPs yet to decide whom they will support.

In return for the Greens' backing, Labor has agreed to a number of concessions on policies and parliamentary procedures. The biggest is a commitment to a cross-party committee on climate change. Members on it must acknowledge that "reducing carbon pollution by 2020 will require a carbon price".

The opposition leader, Tony Abbott, a climate change sceptic, criticised the deal, the first to be done by either of the two main political parties in the hung parliament. "Clearly the Greens will be in the driver's seat of any renewed Gillard government and what this means is there will be a carbon tax," he said.

Gillard acknowledged that the hung parliament required new ways of working but said she did not believe the deal with the Greens would affect Labor's chances of winning the support of the other four independents. "They understand that anybody seeking stable and effective government in these circumstances is going to be talking to a range of people," she said.

Climate change policy has dogged Labor since Kevin Rudd was unable to get his emissions trading scheme through parliament, marking the start of his fall in popularity. During the election campaign, Gillard promised a "citizens' assembly" to forge community consensus on climate change, but the idea was derided. She said she would still work for the assembly through the proposed committee.

The three independent MPs from rural areas have continued briefings with treasury and finance officials. None of them have said whom they will support in a minority government, although they all come from traditionally conservative electorates.

Two of them received a briefing from Lord Nicholas Stern, who is in Australia, on the impact of climate change. He told them a price on greenhouse gases was needed so people could feel the consequences of their actions through the markets.

The rogue Queensland independent MP Bob Katter did not attend the meeting, dismissing Lord Stern's views as "lightweight" with which he disagreed "dramatically".